Brenda and Donna attend an animal rights rally and are crestfallen when the university refuses to suspend animal testing. After Rocky dies of cancer, Brenda becomes involved with a small group of radicals. She serves as lookout when the group breaks into the university research building, and tries in vain to save Andrea's lab (which does not harm animals in its testing). Brenda is arrested. Kelly and Brandon are furious when Dylan refuses to fund Lucinda's film. He finally tells Kelly that Lucinda came on to him. Brandon breaks up with Lucinda, and Kelly confesses her kiss with Brandon. Dylan races over to the Walsh house to fight Brandon, but the tension is defused when he accidentally punches Steve. (At the end of this episode, Tori Spelling thanks the audience and 90210 crew for the making of this 100th episode) . Music: 'Trans-Island Skyway' by Donald Fagen.

At what point did our distant ancestors become anatomically like us? And, more importantly, when did they begin to act like us? Anthropologist Donald Johanson looks at what it is that makes us human.

A scientist becomes the chief suspect when his estranged wife, who's been delaying divorce proceedings, is the victim of a letter bomb.