SpongeBob runs towards Patrick's house, ready to go jellyfishing. Patrick says he'll go with him, but must put away his "secret box" first. Intrigued, SpongeBob tries to peek inside, but Patrick refuses to let SpongeBob see. SpongeBob entices Patrick by sharing his own secrets, but this only convinces Patrick to keep his secret box even more closely guarded. Even SpongeBob's various "secret" possessions fail to distract Patrick from the wonders only he can see.
As Patrick explains the lengths he must go to keep his box safe, he fails to notice SpongeBob sneaking off with it, giddily preparing to look. Patrick stops him just in time and struggles with SpongeBob over the box. The sight of SpongeBob's arms popping off while still desperately clinging to the box disgusts Patrick, making him question the validity of their membership in the Best Friends Forever Club. He threatens SpongeBob with the annulment of their friendship should he persist in trying to sneak a peak. Ashamed, SpongeBob begs Patrick for his forgiveness and promises to respect Patrick's right to secrecy. Placated, Patrick lets SpongeBob off the hook.
That night, SpongeBob cannot sleep, still thinking about Patrick's secret box and what could possibly be inside it. He convinces himself that by sneaking into Patrick's house to look while Patrick sleeps, he'll both spare Patrick's feelings and gain a secret of his own. Despite Gary's admonitions, SpongeBob puts his plan into action, heading out under cover of darkness, pulling on a silk stocking for a mask and slipping under Patrick's rock.
Inside, the box lays in full view on the sleeping Patrick's nightstand. Trying to tiptoe across a floor covered in potato chips and rubber ducks, SpongeBob fears waking Patrick with every step. Finally making it to the bedside, SpongeBob reaches for the box but Patrick sleepily grabs hold of it. SpongeBob gently slips himself in place of the box just as Patrick attempts to take something from it, yanking SpongeBob's tongue in the process. Patrick then places SpongeBob next to him on the bed, allowing SpongeBob to get back onto the floor. As he prepares to leave, SpongeBob trips on his shoelace and begins crashing and bouncing around the rock in a cacophony of noise, culminating in slamming against the wall and breaking a framed photo of him and Patrick in half. Despite all of this, Patrick remains fast asleep.
SpongeBob quietly remarks how heavy a sleeper Patrick is, which causes Patrick to finally wake up and catch the masked SpongeBob with the box. Thinking him to be the Clam Burglar, Patrick prepares to assault him with a pillow until SpongeBob reveals his identity with his Best Friends Forever Ring. In shock and distraught, Patrick declares their friendship to be over. The next moment, however, Patrick changes his mind and allows SpongeBob to go ahead and look. SpongeBob eagerly opens the box, only to find a mere piece of string ("A secret string!" as Patrick describes it). Thanking Patrick for letting him look, SpongeBob heads home, laughing at how he shouldn't have been surprised. Alone once more, Patrick demonstrates the true secret nature of the box: when the string is pulled, a secret compartment opens and reveals to Patrick an "embarrassing picture of SpongeBob at the Christmas party!"