Hotaru Beam is played on a rectangular grid, usually of dashed lines, in which numbers in circles appear at some of the intersections on the grid. Additionally, each circle has a dot on one of the grid lines leading into the circle.
The object is to draw solid lines along the grid lines to connect the circles in a single, contiguous network. The network may not branch or cross over itself, except at a circle. The number in a circle indicates how many turns the network will take when traveling in the direction indicated by the dot before connecting to another circle. For example, a 4 circle with a dot on the right hand-side will have a line coming out of the right-hand side which will turn four times before connecting to another circle.
A zero circle is a given: It will have at least one line going straight in the direction of the dot until it hits another circle. Each circle will have a line coming out of it in the direction of the dot. Since turning at the corner of the grid counts as a turn, a 1 circle near the corner may be forced to simply continue along the edge of the grid.
- Nikoli's description of the puzzle (in Japanese), with example.