Lucy, now played by Hetti Bywater, is back on Albert Square in time for Pat's funeral next week - and takes a disliking to Mandy after discovering that she is her dad Ian's new partner.
Episodes coming up later this month see Lucy hatch a plot against Mandy as they start to spend quality time together - keen to get her in trouble with Ian (Adam Woodyatt).
Stapleton told Soaplife: "Mandy wants to be part of the Beale family and she's desperate to win Lucy over, but she gets it all wrong. She goes to The Vic with Lucy, but because she doesn't want Lucy to see her as the wicked stepmother, she behaves like an older sister.
"Mandy gets drunk and Lucy gets her to buy some drugs. She does it because she thinks it's going to win her points with Lucy.
"Mandy passes out and wakes up to find the pills are gone. It's only later she discovers Lucy's planned it so Bobby finds the drugs."
Asked whether Mandy can avoid Ian's fury by getting herself out of Lucy's frame-up, the actress replied: "Mandy's no fool and I'm loving the storylines I am doing with Adam, so I hope so!"
EastEnders airs Lucy's return on Thursday, January 12 at 7.30pm on BBC One.
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