In early 2011, fans will see Sonia return to Albert Square for one episode as she finds herself drawn into a new storyline centring around her half-sister Bianca Jackson (Patsy Palmer).
Sonia's sudden reappearance in Walford comes after a huge confrontation between Carol, Whitney and Connor sees truths emerge over Connor's relationships with Bianca's mother and stepdaughter.
After events come to a head, Bianca mysteriously disappears from the Square - but Sonia's comeback sees her provide some answers to questions that Carol and Ricky have about the feisty mum's whereabouts.
A Walford insider revealed: "True to form, Bianca has got herself into big trouble while trying to protect her family and, scared about what might happen, she does a runner.
"When she suddenly goes missing, Sonia turns up unannounced and drops the bombshell that she knows where Bianca is and what she is going to do next."
Sonia's brief return will also see some other truths revealed as viewers discover what she and husband Martin have been getting up to away from Walford.
Cassidy's last appearance in EastEnders was in February, when Sonia attended Ricky and Bianca's wedding.