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While Leonard prepares for a three-month science expedition, he and Penny re-evaluate their feelings for each other.

The Big Bang Theory

The Monopolar Expedition

Penny makes a surprising revelation about Leonard while on her date with Stuart; Wolowitz finds a critical design flaw in his NASA project.

The Classified Materials Turbulence

Koothrappali and Leonard take Wolowitz to Las Vegas; Sheldon is forced to bunk with Penny when he gets locked out of his apartment.

The Vegas Renormalization

Wolowitz and Leonard decide to check out a local bar on Ladies' Night.

The Hofstadter Isotope

Penny develops a rivalry with a sexy new female neighbor who threatens her position as queen bee of the building.

The Dead Hooker Juxtaposition

Penny's home business turns into a complete nightmare when Sheldon takes over.

The Work Song Nanocluster

Leonard, Sheldon and Wolowitz are ecstatic to learn that a famous science-fiction actress (Summer Glau) is aboard the train they are taking to San Francisco.

The Terminator Decoupling

A paintball game leads to a fight between Sheldon and Penny, and romance for Leslie and Wolowitz.

The Cushion Saturation

Leonard and Penny are drawn together after a disastrous visit from Leonard's mother (Christine Baranski).

The Maternal Capacitance

Sheldon's solution to Penny's financial difficulty leads to a confrontation between Leonard and Penny's ex-boyfriend.

The Financial Permeability

Sheldon develops a scientific procedure for making friends; Kripke controls a new supercomputer.

The Friendship Algorithm

When Penny's comment about Wolowitz's love life sends him into a depressed stupor, his friends lose their most important teammate in an upcoming fighting-robot competition.

The Killer Robot Instability

Physicists and best friends Leonard and Sheldon learn about the social aspects of life through their relationships with the free-spirited beauty who moves in next door.

S2 E17 | The Terminator Decoupling

E23 | The Monopolar Expedition

E22 | The Classified Materials Turbulence

E21 | The Vegas Renormalization

E20 | The Hofstadter Isotope

E19 | The Dead Hooker Juxtaposition

E18 | The Work Song Nanocluster

E17 | The Terminator Decoupling

E16 | The Cushion Saturation

E15 | The Maternal Capacitance

E14 | The Financial Permeability

E13 | The Friendship Algorithm

E12 | The Killer Robot Instability