Sheldon and Penny bond in Leonard's absence, Raj finds someone to share his heartache with, and while away at an out-of-town conference, Bernadette and Amy are 'hit on' at the hotel bar
Leonard returns early from his research expedition as a surprise for Penny, but Sheldon is upset that he wasn't in on the secret
Raj organises an epic and demanding scavenger hunt for his friends, designed to test their scientific knowledge and common sense, and it brings out some surprising traits in everyone
Amy ruins one of Sheldon's favourite films by pointing out a flaw in the storyline, so he plots revenge by trying to spoil her enjoyment of a much-loved TV show from her childhood
Amy informs Sheldon that she has been asked to work on a scientific project that is taking place at his university
Sheldon is mortified when he stumbles upon an important scientific breakthrough by accident. He feels like a complete fraud and is embarrassed by all the praise and attention that comes his way.
Sheldon feels left out when Professor Proton asks Leonard to read his latest publication, so he befriends a TV personality from a rival science show
Leonard thinks Sheldon will over-react when he admits he failed to return a DVD to the rental shop seven years ago but a surprisingly calm Sheldon finds a way to punish his friend for his past mistake
Howard takes the gang to his mother's house for Thanksgiving in an effort to protect himself from the company of his grumpy father-in-law, who has also been invited
Sheldon continues to feel guilty about the attention he is receiving for his accidental discovery of a new element
Sheldon returns to Texas when his sister goes into labour. While he's away, the gang gather to decorate the Christmas tree and ponder on how things would have been if they'd never met Sheldon.
When Penny's big TV acting break hits the cutting room floor, Leonard does his best to comfort her, which leads her to ask him a bold question about their relationship
When Penny quits her job at the restaurant in order to pursue her acting career full time and Sheldon is forced to take his unused leave by the university, the two end up spending some time together
After the guys miss out on Comic-Con tickets, Sheldon decides to hold his own comic book convention and begins to search for a suitable star guest for the event
Love is in the air when Amy convinces Sheldon to join her - as well as Howard and Bernadette - for a romantic train trip to Napa Valley to celebrate Valentine's Day
When Leonard buys a dining table for the flat, Sheldon objects strongly to such a big change in his living space and his relationship with Amy almost becomes a casualty as a result
In an attempt to improve their strained friendship, Howard invites Sheldon to fly to Houston with him on a visit to Nasa
When Sheldon pays a surprise visit to his mother in Houston, he is forced to confront a new reality
Raj is wracked with guilt about dating Lucy and Emily at the same time, and Penny wrestles with her conscience over whether to accept a role in a terrible horror film
Sheldon faces a personal crisis when he starts to wonder whether he's wasting his time with his study of String Theory, although Penny does her best to offer him advice
Penny and Leonard try to distract Sheldon from his career problems by reinstating Anything Can Happen Thursday
Sheldon struggles to process his feelings of grief when his idol Professor Proton passes away. Leonard turns a relationship milestone into a competition with Penny.
A horrible day at work prompts Penny to evaluate her life choices - including taking a big step with Leonard. Howard and Bernadette struggle to care for Mrs Wolowitz.
With potential changes affecting his professional, domestic and social life, Sheldon is forced to confront an uncertain future and considers a major move, which worries Penny and Leonard