Doctor Strong is the headmaster of David's second, much better, school. Unlike Mr. Creakle, he has no trouble keeping order because all of the students respect and love him. He treats them all like his children, and they respond with an admiring, protective affection that Dickens seems to feel is appropriate for students to have for their teacher.
Always distracted from the world around him by his lofty thoughts, Doctor Strong is like the stereotypical absent-minded professor. His current project is a new dictionary. He's always carrying bits of the dictionary with him so that he can add things as he thinks of them.
Strong is married to a lovely woman much younger than himself named Annie. Annie was actually also originally a student of Doctor Strong's. Doctor Strong had been a friend of her late father. When she left his tutoring, he came to her mother and asked for permission to marry her. Annie was flattered, and agreed.
Because there is such a difference in age between Annie and Doctor Strong, a number of people assume that Annie is having an affair with her cousin, Jack Maldon. Mr. Wickfield makes several ominous statements about people presuming on Doctor Strong's good nature, meaning that he thinks Annie is stepping out on Doctor Strong because Doctor Strong is too nice to suspect her of it. And Jack Maldon, who is expecting Doctor Strong to arrange a job for him, comes out and says plainly that Annie has Doctor Strong wrapped around her little finger, "because Annie's a charming young girl, and the old Doctor – Doctor Strong, I mean‚ is not quite a charming young boy" (16.55).
These suspicions all come to a head after David has left school and become a proctor, when the Wickfields come to London on business, bringing Uriah Heep along with them. Once Doctor Strong has retired from teaching and become a full-time dictionary-writer, he moves with Annie to a house in Highgate (not far, incidentally, from Mrs. Steerforth's). So the Wickfields, David, and Uriah Heep all troop over to Highgate to visit Doctor Strong. There, Uriah Heep finally tells Doctor Strong what Doctor Strong has always been too distracted and generous to notice: the whole world thinks that Annie has been cheating on him.
Instead of making Doctor Strong angry, as Uriah Heep expects, this rumor makes him sad. He is sure that, if Annie is having an affair, it's his fault for marrying such a young lady and for being too boring for her. So, Doctor Strong refuses to ask Annie about these suspicions. He doesn't want her to know that the neighborhood doubts her reputation. He also swears David and Mr. Wickfield to secrecy about these rumors. Everything is at a standstill: Doctor Strong is angry at himself because he thinks he's holding Annie back, so he tries to encourage her to go out and have as much fun as she can. And Annie is feeling miserable because Doctor Strong keeps pushing her away.
Who should step in to solve this problem but Mr. Dick? Mr. Dick has remained close to Doctor Strong since David's school days. When Miss Betsey loses all of her money, David takes on an extra job working for Doctor Strong as a secretary. As David goes every morning to Doctor Strong's house for work, Mr. Dick regularly accompanies him so that he can visit the Strongs.
Mr. Dick notices that there is distance between the Doctor and his wife, but he realizes that David and Miss Betsey would find it socially awkward to intervene directly. So, Mr. Dick decides to help. One day, when Mr. Dick, Miss Betsey, and David are visiting Doctor Strong, Mr. Dick sneaks off to find Annie in the garden. Doctor Strong is in his study.
Mrs. Markleham, Annie's mother, greets David and tells him that Doctor Strong is busy writing his will. Mrs. Markleham bursts into Doctor Strong's study to thank him for thinking so generously of her daughter. Just then, Mr. Dick leads Annie in to Doctor Strong's study. Annie finally confronts Doctor Strong.
Annie's side of the story is this: she married Doctor Strong in good faith, because she admired and loved him. But her scheming mother has been using Doctor Strong as a meal ticket to get jobs for all of Annie's poor relations (including Mr. Jack Maldon). Out of shame for her mother's exploitation of Doctor Strong, Annie began to draw away from him.
Annie knew that the neighborhood could only assume that she had married Doctor Strong for her own profit. But it's not so! She and Jack Maldon may have been childhood sweethearts, but they really have nothing in common. It's only Doctor Strong who has been the true companion of her soul. Doctor Strong comes to realize his mistake, he embraces Annie, and all is well – thanks to Mr. Dick's excellent timing!
There's a lot of noteworthy language blurring the lines of father and husband here in the relationship between Doctor Strong and Annie. David observes that Doctor Strong has a "fatherly, benignant way of showing his fondness for [Annie]" (16.119). And later, Annie herself describes Doctor Strong's "dear face" as one "revered as a father's, loved as a husband's, sacred to me in my childhood as a friend's" (55.132). Annie is perfectly willing to submit herself to Doctor Strong's greater knowledge, and to help and support him as best she can in his scholarship. This marriage does have its own problems (like the Jack Maldon disaster) but it is fundamentally strong.
Doctor Strong's marriage provides an interesting counterpoint to Mrs. Copperfield and Mr. Murdstone's, and even to David and Dora's. Like Doctor Strong, Mr. Murdstone wants to be a fatherly figure to his wife, training and guiding her to be better. But he's such a brute that he makes her life a misery. And like Annie, Dora wants David to be like a father figure to her, going so far as to ask him to think of her as his "child-wife." But Doctor Strong's marriage seems to work only because both parties are satisfied with their places in the marriage; unless you're a gentleman, like David or Traddles, satisfaction with your lot in life seems to be one of the big lessons of this book.