Oobi and Uma make a valentine for Grampu. They receive a "mystery valentine" from the letter carrier and try to figure out who the mystery valentine is. They follow a trail of hearts to discover their mystery valentine. — NickJr.com (source)
Oobi is in his room, putting a Valentine's Day card together. He shows his card to Uma, who asks if she can help. Oobi lets her choose the shape to place on the front of the card. She picks a pink heart. After Oobi finishes the card, he gives it to Uma.
Uma thanks him and decides to create on a card for Grampu. The next scene shows Grampu sewing a festive Valentine's Day costume together. He puts it away when Oobi and Uma walk up to him. The children present Grampu with their card, and he thanks them both. The family walks to the front door after hearing the doorbell ring.
A mailman named Cliff is shown at the door, holding a heart-shaped paper. Grampu takes the paper, thanks Cliff, and reads it aloud to his grandchildren. It is addressed to them from a "mystery valentine" and states that a trail of heart-shaped cards will lead to the sender. Oobi and Uma become determined to figure out who sent them the message.
They spot a card in the front yard, where Kako greets them. He is carrying many cards, two of which he gives to Oobi and Uma. They ask Kako if he is their mystery valentine, but he assures them that he is not. He leaves to deliver the rest of the cards to his friends. As they say "goodbye" to Kako, Oobi and Uma notice another card by the windowsill. They walk over to it and are interrupted by a delivery woman, who has a letter and a bouquet of balloons for Grampu.
Grampu appears and tells the children that the note is from Inka. He sighs lovingly as he takes the letter inside. As Oobi and Uma watch Grampu, they find another card in a nearby tree. It leads them to the backyard. While they continue searching, an interview segment is featured. It involves Kako talking to preschoolers about Valentine's Day.
The scene cuts to Oobi and Uma again, who have found another card on the back door. They open it and see Grampu inside. He is dressed in the heart-shaped costume from earlier. Grampu wishes Oobi and Uma a happy Valentine's Day as Oobi says "goodbye" to the viewers, ending the episode.