The movie Dirty Grandpa is not a sequel to Bad Grandpa.
In the movie Dirty Grandpa, right before his wedding, an uptight guy (Zac Efron) is tricked into driving his grandfather (Robert DeNiro), a perverted, foul-mouthed former Army general, to Florida for spring break.
This movie has been done before. Most recently, Melissa McCarthy did it with Tammy. Lily Tomlin was Grandma and Barbra Streisand did it in The Guilt Trip.