A method for the synthesis of 8-fluoro-5-(4-fluorobenzyl)-3-(2-methoxybenzyl)-3,5-dihydro-4H-pyrimido[5,4-b]indol-4-one has been developed and the electronic and spatial structure of a new biologically active molecule has been studied both theoretically and experimentally. The title compound was crystallized from acetonitrile and the single-crystal X-ray analysis has revealed that it exists in a monoclinic P21/n space group, with one molecule in the asymmetric part of the unit cell, a = 16.366(3) Å, b = 6.0295(14) Å, c = 21.358(4) Å, β = 105.21(2)°, V = 2033.7(7) Å3 and Z = 4. Hirshfeld surface analysis was used to study intermolecular interactions in the crystal. Molecular docking studies have evaluated the investigated compound as a new inhibitor of hepatitis B. Testing for anti-hepatitis B virus activity has shown that this substance has in vitro nanomolar inhibitory activity against Hepatitis B virus (HBV).