Things take a turn for the worse when Johnny gives his friend Frankie an unauthorized tour of Reed's lab. When Frankie is sucked into another dimension, Johnny goes in after her to prevent anyone from finding out. But as he does so, a small bug comes through the dimensional portal. Susan, Ben, and Reed now have to deal with the bug, while Frankie and Johnny are trapped in the other dimension.

Dr. Doom causes an energy surge during one of Reed's experiments. As a result the Four have swapped powers with one another. Now the team must face Doom with new and untrained skills.

The Four have a contest called "Fantastic Fifth for a Day." But when the Skrulls return with the Super Skrull, it may be up to nerdy teen Rupert, the Fantastic Fifth, to save the day.