“Frontline” (8 p.m., PBS, check local listings) enters a new season with “Cheney’s Law,” a look at Vice President Dick Cheney’s decades-long concern with the erosion of presidential powers during the Vietnam-Watergate era and his efforts to reassert those powers over the past six years.

“Law” contains extensive interviews with two Justice Department lawyers who were once rising stars in conservative legal circles. In the hours and days following the attacks of Sept. 11, John Yoo drafted memoranda that gave the president unprecedented wartime powers. “Frontline” portrays Yoo as very much under the influence of the vice president’s office and Cheney’s lawyer Richard Addington.

Yoo continues to defend his philosophy and argues that critics have not yet come up with alternatives that would protect the country against possible terrorists. But when his friend, colleague and fellow conservative legal scholar Jack L. Goldsmith replaced Yoo in the White House’s Office of Legal Counsel, he was shocked at Yoo’s legal handiwork. He felt that the Cheney-Addington-Yoo philosophy of unfettered presidential power was legally flawed and indefensible under the American Constitution.

Goldsmith would soon tangle with the administration over issues, including granting Geneva Convention rights to Iraqi prisoners, the definition of torture and the wiretapping of American civilians.

The drama reaches a bizarre moment right out of “24” when then-White House counsel Alberto Gonzales and presidential adviser Andrew Card raced to the hospital bedside of a very ill John Ashcroft to try to get the enfeebled attorney general to sign off on the extension of a surveillance program that Ashcroft had earlier declared illegal. Goldsmith recalls fearing that Ashcroft was “near death,” but he sat up and “read them the riot act” instead.

“Frontline” also chronicles more recent clashes between Congress and the White House, most notably the proliferation of “signing statements,” attached to laws passed by Congress and signed by the president that assert the executive’s right to disregard the spirit and letter of the law. So if the president can ignore Congress, do we still have three equal branches of government? Or have the checks and balances that the founders intended become a thing of the past?

“Frontline” declares that the president has added hundreds of such statements to signed bills and that Richard Addington crafted most of them.

Both Cheney and Addington declined an invitation to appear. “Frontline” gives Yoo free rein to defend and explain his legal philosophy. But the consistent use of photographs showing a grim and resolute vice president as well as ominous background music and sound effects create the impression of a stacked deck and undercut the seriousness of this compelling documentary.

¢ The 2006 documentary “Wordplay” on “Independent Lens” (9 p.m., PBS, check local listings) celebrates the joys of crossword puzzles.

¢ Are we still being affected by the environment and diet that shaped our grandparents? “Nova” (7 p.m., PBS, check local listings) explains on “The Ghost in Your Genes.”

Tonight’s other highlights

¢ Sam receives a white dove on “Reaper” (8 p.m., CW).

¢ Alex takes a divisive stance on “Cane” (9 p.m., CBS).

¢ The only eyewitness to a brutal attack is the victim’s mentally challenged daughter on “Law & Order: Special Victims Unit” (9 p.m., NBC).

¢ Denny’s pal has a big announcement on “Boston Legal” (9 p.m., ABC).

¢ Ellen’s plight explained on “Damages” (9 p.m., FX).