This is a Phase II, open-label, single-arm, multicenter study of intravesical BCG plus ALT-803 in patients with BCG unresponsive high grade non-muscle invasive bladder cancer (NMIBC). All patients treated in the study will receive via a urinary catheter in the bladder, BCG plus ALT-803 weekly for 6 consecutive weeks (initial induction treatment period). After the first disease assessment, eligible patients will receive either a 3-week maintenance course or a 6-week re-induction course (second treatment period) at Month 3. Eligible patients will continue to receive maintenance treatment in the third treatment period at Months 6, 9, 12, and 18. The study duration is 24 months.