She stacked Doctor Who DVDs on shelves at Blockbuster, but Freema Agyeman has since entered the Tardis, becoming the Timelord’s new assistant.

The 27-year-old replaces Billie Piper when the series returns this month. After years of audition disappointments, the little-known actress is playing the medical student Martha Jones. The Doctor and the medic are thrown together when the Royal New Hope Hospital in London is transported to the Moon, and the pair are soon battling a galactic clan.

Agyeman, who was brought up in North London by her Iranian mother and Ghanaian father, is about to become one of the most recognisable faces on British television. She has already been modelled in plastic for a Martha action figure.

Agyeman had a minor role in last year’s series and believed she was auditioning for a role in Torchwood when she was summoned to meet the writing team in Cardiff. “I was absolutely elated,” she said after the truth was revealed. “I hadn’t allowed myself to entertain the thought that I would get it.”

Martha is expected to be a “sassier” sidekick than her predecessor, Rose Tyler. Piper has since become an acclaimed West End actress.

David Tennant, who returns in the title role, said: “There is a different dynamic, because the Doctor is not looking for a new best friend. Or, at least, he thinks he isn’t.”

The Doctor and Martha are destined to meet Shakespeare, alien Plasmavores and a sinister intelligence at work in 1930s New York, where the Daleks are once again ready to wreak havoc. Tennant said: “The scale is bigger, the locations are more extraordinary.”

Phil Collinson, the producer, said: “The biggest pressure is coming up with new, different and scary monsters. This year there’s probably a bigger selection.”

Agyeman worked shifts at a video shop so that she could get time off for auditions. Her casting will help to stem criticism from those, Jonathan Ross among them, who questioned the BBC’s commitment to putting ethnic minorities in leading roles. Martha is the first black assistant in the show’s 43-year history.