Basil's mother, Brenda, was considered half-crazy. She called herself Queen of the Fairies and thought her offspring were demonic changelings, left by evil fairies as substitutes for her real children. Basil (1909-45), the fourth Marquess, was educated at Lockers Park, a "prison-like" prep school in Hertfordshire, won the prestigious Rosebery History Prize at Eton and was a Brackenbury Scholar at Balliol College. A friend described his "beautiful brown eyes, very very alive and deep and large", and said he was quiet and reserved, with fine manners. The biographer Elizabeth Longford called Basil "brilliantly clever . . . very well grown, extremely handsome, very athletic". Lord Birkenhead, another contemporary, recalled "his incessant chucklings at his own sallies, his dark and striking face thrown forward in fierce argument". Evelyn Waugh admired him. John Betjeman fell in love with him, and gave him the affectionate schoolboy nicknames of "Little Bloody" and the ironic "Mindless". He noted his lively conversation and described his appearance and character: "Lord Ava had enormous eyes / And head of a colossal size / He rarely laughed and only spoke / To utter some stupendous joke." Betjeman later wrote an elegy that described him as "Humorous, reckless, loyal / My kind, heavy-lidded companion".

Basil's intellect matched his beauty and character. The Oxford economist Roy Harrod praised him as "the most brilliant pupil I ever had". Randolph Churchill, agreeing with Betjeman and Harrod, lauded Basil as "the most lovable man I met at Oxford. His liquid spaniel eyes and his beautiful, charming manner commanded affection. He was the most brilliant of all my contemporaries at Oxford." But Churchill, himself a heavy drinker, added, "an undue addiction to drink blighted what might have been a fine political career". Basil was also addicted to gambling, and heavily mortgaged his vast estate to pay his gaming debts. He solved his financial problems in July 1930 by marrying his cousin, Maureen Guinness, heiress to the inexhaustible brewery fortune. Basil succeeded to his title that month, and spent his honeymoon traveling in Burma and shooting big game with the Maharajah of Mysore in India.

Basil's three young children had only the vaguest memories of their busy and often absent father. His son, Sheridan, observing him in the bath when he was on leave from India, was startled to find him covered with swollen mosquito bites. Caroline's most vivid memory was accompanying her father on a massacre of pheasants and being sent out to fetch the dead birds. Her younger sister, Perdita, given a box of rare wartime chocolates by an American soldier, looked forward to eating them. But when she left them in the library, her father succumbed to temptation and devoured the entire box.

Basil had a dazzling political career and held a series of important government posts in the 1930s. He was private secretary to Lord Lothian, the Under-Secretary of State for India, in 1932; to Lord Halifax, President of the Board of Education, 1932-5 and Secretary of State for War, 1935 and Lord Privy Seal, 1935-36. Basil himself became Under-Secretary of State for Colonies from 1937 to 1940. An effective public speaker, deftly mixing argument and wit, he made his maiden speech in the House of Lords at the age of 22. The historian William Maguire, who did not think Basil's drinking was an impediment, described him as "a gifted young man of extraordinary charm; like his grandfather [the viceroy] he combined intellectual, literary and artistic gifts with ambitions in public life, and as Under-Secretary of State while still in his twenties, he was talked of in some circles as a future Conservative prime minister".