Martha and the Thief of Hearts is the episode after Itchy Martha.
It begins on a snowy Valentines' Day, when Carolina, Helen, and T.D. are walking to Helen's place. Carolina asks how many Valentines her friends got. They got nineteen, but Helen says that's only because everyone in their class had to give each other a Valentine. Carolina says she got twenty-eight.
At Helen's house, Martha asks Helen to read her Valentine (that she got with many others) that has a picture of Courageous Collie Carlo on it. Helen reads "Martha, you're the hound of my heart" and adds that it's from Alice. Truman, who's also visiting, says that Martha is popular. Martha says that Skits got one more than her, but that's because she herself sent him one. Carolina then shows off her Valentine which sings, however, nobody likes it (except Martha) as it sounds like Carolina's own voice (and probably is since she said she sent it to herself) and doesn't scan right. T.D. then serves all the humans (including himself) hot chocolate with marshmallows. Carolina then boasts that the Valentine she made is original. Helen agrees and says that hers are mostly quite boring as she got too many ones with the same illustration of a pickle that says "You mean a great dill to me". Nobody likes that joke. T.D. says that most people just buy the same box and don't bother to personalise them. Carolina says she could make him a personalised Valentine that would be exquisite. T.D. thinks that means it squirts water and when Carolina defines its actual meaning, he says he would prefer one that squirts. Skits barks at the door and Martha asks if anyone would throw snowballs for Skits and she would do it herself, but has no thumbs. She also asks for someone to open an envelope.
Outside, Truman reads a Valentine to Martha which reads "Canine, be mine". T.D. says he drew it and Martha thanks him. Helen gets ready to throw a snowball to Skits, but T.D. wants to draw more Valentines and invites Truman (who looks a bit doubtful) in to make them. Helen throws Skits' snowball and then follows them.
Inside, Carolina has misplaced her eraser. T.D. enters but Carolina thinks he's looking at the Valentine and will spoil the surprise or copy her. He says he just needs a place to draw (and he doesn't like the glitter). Helen finds Carolina's eraser and Carolina angrily tells T.D. to go work in the other room. He goes off, then Carolina tells Helen and Skits to buzz off too.
In Helen's bedroom, Truman admires Helen's art supplies. Helen takes some scraps of fabric to decorate her Valentines.
In the lounge, T.D. explains the meaning of "personalise" to Martha, who thinks it means something is just for people. Carolina enters, having lost her glitter pen and asks T.D. if he took it, but he doesn't know why he'd want it. Helen enters, asking if there's a problem, and Martha, annoyed, says that Carolina cannot find her pen. Carolina asks Helen if she took it, but Helen says no, but says she might have one upstairs. Carolina is shocked because she notices Helen has glitter on her hands. She asks T.D. if he saw it, but he says that she probably just got it when she was looking under the table because there was "a major sandstorm of glitter" there. He then goes to look for Carolina's pen under the table.
In Helen's room, Truman is looking nervously at some blank paper with a bottle of glue in one hand and a pair of scissors in the other. Helen asks if he can't decide what to do, to which Truman replies that it's important to perfect the design. He then goes on to define the meaning of "design", while Helen searches for a glitter pen. Truman then (in a scared, slightly sad voice) tells Helen he can't do his Valentines as he is scared and doesn't know what to do. Helen says it's just a Valentine, to which Truman replies that everyone says that it's just a Valentine, but that's to cover up the fact that it's actually craft. Helen says that it's fun and you can use whatever you want to decorate it, but Truman says "That's craft!! That's the dictionary definition of craft!", and recites the dictionary definition of craft. Helen asks what's wrong with craft, and Truman, with a serious expression, tells her something he's never told anyone else. It happened in kindergarten when everyone was doing crafts and he'd accidentally glued his head to the table. He explains that the reason he doesn't want to make Valentines is partly because he has no inspiration and partly because he's scared he will accidentally glue scissors to his head. They go downstairs, Truman still scared, and Helen gives him a candy heart and tells him to just draw something. He eats it and then goes to look at T.D.'s notebook.
In the kitchen, T.D., Skits, and Carolina are looking under the table. Carolina insists there was glitter on Helen's hand (which there was, but that's not the point). T.D. says that there is a tonne of glitter under the table but no glitter pen. Helen enters and asks what they are looking for and, for some reason, T.D. lies that they are just playing with Skits. Helen says she can't find the pen and Carolina agrees for the sake of being friendly but doesn't seem to buy it, as evidenced when she and T.D. are by the stairs and she says that it's ironic that she was making a Valentine about Helen being wonderful, when she's (allegedly) taking her pen. T.D. says that Helen doesn't steal, but then notices his notebook is gone.
They run in and T.D. says that it has in it his sketches for Valentines, his pirate name ideas, his old gum, and other things which he considers "important". Helen enters, looking for her fabric, which nobody has seen. After searching the attic, Helen's bedroom, and the kitchen (where Truman is busy scribbling) and not finding the art supplies, T.D. suggests that they are dealing with someone who is stealing them to stop them from making Valentines. Skits comes in and barks, with Martha translating that to mean that he has something to show them. They go outdoors, but Skits just wants them to throw more snowballs.
When they enter, Helen finds a Valentine which reads "You stole my heart and T.D.'s notebook, and Carolina's glitter pen, and Helen's fabric." Helen, after reading it, asks who wrote it. Truman looks at it and recognises the writing as Carolina's. Carolina admits she wrote it but says it's because she thought it would make whoever took their art supplies confess.T.D. then stands in front of the archway separating the living room and kitchen and dynamically says "Nobody leaves this room! We'll get to the bottom of this!". Carolina says she should be the one who disallows anybody to leave the room and T.D. angrily asks why. Carolina, also angry, says in a sassy voice that it's because she didn't steal the art supplies. This offends T.D. Carolina says that she will bring everyone's parents into the deal, which scares Truman and she points that out. Martha asks if they can make cards without the missing art supplies. Carolina, still angry, says that she could but one can't make cards without trusting the receiver. This angers T.D., Truman, and Helen who start angrily talking (but not shouting) in unison. Martha yells "Quiet!" and adds that she knows who did it and leads them into the living room.
In the living room, Martha stands on the floor, Carolina and Truman sit on the couch, Skits lies on the floor, Helen sits on the floor, and T.D. sits on Martha's favourite chair. Martha asks why anybody would want to steal their supplies. T.D. says because they hate art supplies. Martha asks what the thief stole from him, and he replies that they stole his notebook. Carolina says that everyone except Truman was in the kitchen when the notebook disappeared. Martha asks Truman where he was. Truman says he was there and admits he was looking into T.D.'s notebook for inspiration and to overcome his fear. T.D. accuses Truman of taking his notebook and demands it back. Truman says that he didn't take it and why would he? Martha says that he would not and asks Carolina if he was around when her glitter pen went missing. She says no. Helen asks why Martha thinks the art supplies were stolen and she asks Truman to look under the couch cushion. He does, revealing the missing art supplies. Carolina asks who did it and Martha says it was the only one who has the motive and opportunity, plus always hides things under the cushion. T.D. finds a page in his notebook covered in slobber. Helen concludes it was Skits who was doing it, and Martha says it was because he was jealous. Helen strokes his cheek, letting him know they are not mad. Then, they go outside to throw snowballs for Skits. They do, and T.D. builds a huge snowball but can't pick it up.
- It is unknown why T.D. lied about playing with Skits while looking for Carolina's pen because there is nothing shameful about looking for it.
- The "you stole my heart" beginning of Carolina's note was in the first person (to apply anonymity), and was probably designed to both soften the blow and make it sound more like a valentine.